(1) Field of the Invention
The present invention relates to a process for obtaining carbon black from an aqueous suspension by mixing the suspension with low-boiling hydrocarbons, then expanding the mixture into a vessel, whereby the mixture separates into a gas phase of the hydrocarbon vapours which form during expansion and carry the carbon black, and into an aqueous liquid phase, and recovering the carbon black from the gas phase.
(2) Description of the Prior Art
It is known to hydrophobisize carbon black suspended in water, as produced, for example, in oil-pressure gasification processes, by admixing hydrocarbons, which causes the carbon black to separate from the water. The general purpose of this process is to re-supply the separated carbon black to the gasification reactor and to re-use the water freed from carbon black in the process in which the carbon-black-water-suspension is produced. In these cases, it is appropriate to carry out hydrophobisization directly with heavy hydrocarbon oils, e.g. using heavy fuel oils or crude oil vacuum residues. Since the separation process is relatively slow when mixing with heavy oils, even if the process is carried out at elevated temperatures, it has proved successful to carry out hydrophobisization using low boiling hydrocarbons and in a second stage, to mix the carbon black-hydrocarbon-suspension with a heavy oil. After the light hydrocarbons have been separated by distillation, the suspension may then be returned to the gasification installation. If, however, the carbon black is to obtain as such, then, according to the prior art, the suspension of carbon black in liquid hydrocarbons is separated from the carbon black-free water and the hydrocarbon is separated mechanically or by evaporation.
The carbon black obtained in this manner still contains large quantities of water, e.g. as much as 15% (DT OS No. 2,216,155), and even more according to the method of recovering, e.g. from 40% to 70% (DT OS No. 2,510,876). If carbon black having a lower water content is required, then an expensive drying process must be carried out subsequently which generally takes place at temperatures of from 35.degree. to 600.degree. C.
In a process recently described carbon black may be obtained from aqueous carbon black suspensions having a water content of only approximately 2% by weight. This is achieved by treating the aqueous carbon black suspension with low-boiling hydrocarbons at elevated pressure and then expanding the mixture, during which expansion, a separation into evaporated hydrocarbons, water which is extensively freed from carbon black and carbon black takes place (DT OS No. 2,546,072).